Hashimoto T, Shoji T, Mihara T, Oguri H, Tamaki K, Suzuki K, Yamada Y
Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Japan.
Plant Mol Biol. 1998 May;37(1):25-37. doi: 10.1023/a:1005961122814.
The putrescine N-methyltransferase (PMT) cDNA clone previously isolated from tobacco encodes a spermidine synthase-like protein with an 11 amino acid element repeated four times in tandem at the amino terminus. Genomic Southern blot analyses indicated that this N-terminal repeat array is found in tobacco PMTs but absent in Hyoscyamus and Atropa PMTs. A truncated tobacco PMT in which this repeat array was entirely removed still retained full enzymatic activity when expressed in Escherichia coli. Three PMT genes (NsPMT1, NsPMT2, NsPMT3) isolated from Nicotiana sylvestris encode two, five, and nine tandem repeats, respectively, in the first exon, but otherwise encode highly conserved proteins. Analysis of PCR fragments amplified from the genomes of N. tabacum and its two probable progenitors shows that one of the nine repeat elements in NsPMT3 was precisely deleted in the corresponding N. tabacum gene. These results indicate that direct tandem repeats of a 33 bp sequence that encodes 11 amino acids of no obvious function were added to the ancestral Nicotiana PMT gene, and that the tandem repetition was genetically very unstable, contracting or expanding during evolution of the Nicotiana species.
先前从烟草中分离出的腐胺N-甲基转移酶（PMT）cDNA克隆编码一种类似亚精胺合酶的蛋白质，其氨基末端有一个11个氨基酸的元件串联重复了四次。基因组Southern杂交分析表明，这种N末端重复序列存在于烟草PMT中，但在天仙子和颠茄PMT中不存在。当在大肠杆菌中表达时，一个完全去除了该重复序列的截短烟草PMT仍保留了全部酶活性。从林烟草中分离出的三个PMT基因（NsPMT1、NsPMT2、NsPMT3）在第一个外显子中分别编码两个、五个和九个串联重复序列，但编码的是高度保守的蛋白质。对从烟草及其两个可能的祖先基因组中扩增的PCR片段进行分析表明，NsPMT3中的九个重复元件之一在相应的烟草基因中被精确删除。这些结果表明，一个编码11个无明显功能氨基酸的33bp序列的直接串联重复序列被添加到了烟草属祖先的PMT基因中，并且这种串联重复在遗传上非常不稳定，在烟草属物种进化过程中会收缩或扩展。
